Move docs test workflow and rm obsolete docs processes

[pedro-psb]

This does:

• Adds a docs test workflow in the ci.yml
• Removes docs test from test.yml workflow (which tests the plugin)
• Deprecate --docs feature from plugin-template
  • remove docs_test option
  • remove template/docs
  • remove template/bootstrap/docs legacy content
  • remove ci_update_docs
  • make it no-ops
• Update various info across files

Closes: #901

Testing

This does not cover all workflows, only CI

• Run in a sample PR: https://github.com/pedro-psb/pulp_python/pull/2/checks
• Merging to non-main branch: https://github.com/pedro-psb/pulp_python/pull/3/checks
• Cache test: Not very meaningful time saving. Around 30mb of cached data
  • no-cache run
  • cached run

[mdellweg]

Let's merge and continue from there.